Origin of weak ferromagnetism in Yb_xFe₄Sb ₁₂, relationship between weak ferromagnetism and filling ratio x

The magnetization and specific heat of single-crystalline samples of filled-skutterudite compounds Yb_xFe₄Sb₁₂ were measured. Six single crystals with the filling ratio of Yb, x, from 0.875 to 0.910 were prepared. The weak ferromagnetism of Yb_xFe ₄Sb₁₂ appearing at low temperatures was found to strongly correlate with x; with increasing x from 0.875 to 0.910, the transition temperature, T_C, decreases from 17 to 5K and the weak-ferromagnetic moment, m₀, decreases from 0.066 to 0.005μ_B/Fe. From these trends, both T_C and m₀ were expected to reach zero at the critical concentration, x_c = 0.93. In the temperature dependence of the specific heat, no anomaly at T_C was observed, reflecting that m₀ is extremely small compared with the effective Bohr magneton number. With the application of pressure, the TC of the sample with x = 0.882 increases rapidly, with the ratio of dT_C/dP = +5:27 K/GPa. This weak ferromagnetism of Yb_xFe₄Sb₁₂ originates from the itinerant 3d electrons of Fe in anions [Fe ₄Sb₁₂].